In terrestrial ecosystems, plants take up phosphate predominantly via association with arbuscular mycorrhizal fungi (AMF). We identified loss of responsiveness to AMF in the rice (Oryza sativa) mutant hebiba, reflected by the absence of physical contact and of characteristic transcriptional responses to fungal signals. Among the 26 genes deleted in hebiba, DWARF 14 LIKE is, the one responsible for loss of symbiosis . It encodes an alpha/beta-fold hydrolase, that is a component of an intracellular receptor complex involved in the detection of the smoke compound karrikin. Our finding reveals an unexpected plant recognition strategy for AMF and a previously unknown signaling link between symbiosis and plant development.

The plasma membrane (PM) is composed of heterogeneous subdomains, characterized by differences in protein and lipid composition. PM receptors can be dynamically sorted into membrane domains to underpin signaling in response to extracellular stimuli. In plants, the plasmodesmal PM is a discrete microdomain that hosts specific receptors and responses. We exploited the independence of this PM domain to investigate how membrane domains can independently integrate a signal that triggers responses across the cell. Focusing on chitin signaling, we found that responses in the plasmodesmal PM require the LysM receptor kinases LYK4 and LYK5 in addition to LYM2. Chitin induces dynamic changes in the localization, association, or mobility of these receptors, but only LYM2 and LYK4 are detected in the plasmodesmal PM. We further uncovered that chitin-induced production of reactive oxygen species and callose depends on specific signaling events that lead to plasmodesmata closure. Our results demonstrate that distinct membrane domains can integrate a common signal with specific machinery that initiates discrete signaling cascades to produce a localized response.

In plants, antimicrobial immune responses involve the cellular release of anions and are responsible for the closure of stomatal pores. Detection of microbe-associated molecular patterns (MAMPs) by pattern recognition receptors (PRRs) induces currents mediated via slow-type (S-type) anion channels by a yet not understood mechanism. Here, we show that stomatal closure to fungal chitin is conferred by the major PRRs for chitin recognition, LYK5 and CERK1, the receptor-like cytoplasmic kinase PBL27, and the SLAH3 anion channel. PBL27 has the capacity to phosphorylate SLAH3, of which S127 and S189 are required to activate SLAH3. Full activation of the channel entails CERK1, depending on PBL27. Importantly, both S127 and S189 residues of SLAH3 are required for chitin-induced stomatal closure and anti-fungal immunity at the whole leaf level. Our results demonstrate a short signal transduction module from MAMP recognition to anion channel activation, and independent of ABA-induced SLAH3 activation.

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In plants, a variety of stimuli trigger long-range calcium signals that travel rapidly along the vasculature to distal tissues via poorly understood mechanisms. Here, we use quantitative imaging and analysis to demonstrate that traveling calcium waves are mediated by diffusion and bulk flow of amino acid chemical messengers. We propose that wounding triggers release of amino acids that diffuse locally through the apoplast, activating the calcium-permeable channel GLUTAMATE RECEPTOR-LIKE 3.3 as they pass. Over long distances through the vasculature, the wound-triggered dynamics of a fluorescent tracer show that calcium waves are likely driven by bulk flow of a channel-activating chemical. We observed that multiple stimuli trigger calcium waves with similar dynamics, but calcium waves alone cannot initiate all systemic defense responses, suggesting that mobile chemical messengers are a core component of complex systemic signaling in plants.

14The plasma membrane (PM) that lines plasmodesmata has a distinct protein and lipid 15 composition, underpinning specific regulation of these connections between cells. The 16 plasmodesmal PM can integrate extracellular signals differently from the cellular PM, but it 17 is not known how this specificity is established or how a single stimulus can trigger 18 independent signalling cascades in neighbouring membrane domains. Here we have used the 19 fungal elicitor chitin to investigate signal integration and responses at the plasmodesmal PM. 20 We found that the plasmodesmal PM employs a receptor complex composed of the LysM 21 receptors LYM2 and LYK4 which respectively change their location and interactions in 22 response to chitin. Downstream, signalling is transmitted via a specific phosphorylation 23 signature of an NADPH oxidase and localised callose synthesis that causes plasmodesmata 24 closure. This demonstrates the plasmodesmal PM deploys both plasmodesmata-specific 25 components and differential activation of PM-common components to independently 26 integrate an immune signal. 27 28 93that plasmodesmata are regulated independently of other immune responses, suggesting that 94 there is a critical requirement for a cell to finely tune connectivity to its neighbours. 95 4 Results 96Chitin-triggered plasmodesmata closure is dependent on LYK4 and LYK5 97 We previously identified that LYM2 is a GPI-anchored, LysM receptor protein that is 98 resident in the plasmodesmal PM (Faulkner et al., 2013). As LYM2 has no intracellular 99 domains we reasoned that it must interact with other proteins to initiate downstream signals 100 that result in plasmodesmal responses. Ligand perception by LysM RKs and RPs often 101 involves multiple members of the LysM protein family: chitin perception in rice involves 102 both the RP CHITIN ELICITOR BINDING PROTEIN (OsCEBiP) and the RK CHITIN 103 ELICITOR RECEPTOR KINASE 1 (OsCERK1) (Kaku et al., 2006; Hayafune et al., 2014); 104 peptidoglycan perception in Arabidopsis involves the RK CERK1, and RPs LYM1 and 105 LYM3 (Willmann et al., 2011); and PM chitin perception in Arabidopsis involves CERK1 106 (also called LYK1) and the RKs LYK4 and LYK5 (Cao et al., 2014). Thus, we hypothesised 107 that LYM2 might partner with a LysM RK for signalling. The Arabidopsis LysM RK family 108 consists of 5 members: CERK1/LYK1, LYK2, LYK3, LYK4 and LYK5. To narrow down 109 plasmodesmata signalling candidates we screened publicly available data sets for LYK gene 110 expression. Comparing data sets from seedlings (GSE74955, Yamada et al., 2016; 111 GSE78735, Hillmer et al., 2017) and mature leaves (eFP browser, Winter et al. 2007) we 112 identified variable expression patterns for the LYK family members (Fig. S1). Thus, we 113 performed RT-PCR to identify members of the family expressed in mature Arabidopsis 114 leaves where we assay for and detect LYM2 function. Only transcripts from CERK1, LYK3, 115LYK4 and LYK5 were detected in mature leaves grown in our conditions, eliminating LYK2 116 f...

SummaryMulticellular organisms exchange information and resources between cells to co-ordinate growth and responses. In plants, plasmodesmata establish cytoplasmic continuity between cells to allow for communication and resource exchange across the cell wall. Some plant pathogens use plasmodesmata as a pathway for both molecular and physical invasion. However, the benefits of molecular invasion (cell-to-cell movement of pathogen effectors) are poorly understood. To begin to investigate this and identify which effectors are cell-to-cell mobile, we performed a live imaging-based screen and identified 15 cell-to-cell mobile effectors of the fungal pathogen Colletotrichum higginsianum. Of these, 6 are “hypermobile”, showing cell-to-cell mobility greater than expected for a protein of its size. We further identified 3 effectors that can indirectly modify plasmodesmal aperture. Transcriptional profiling of plants expressing hypermobile effectors implicate them in a variety of processes including senescence, glucosinolate production, cell wall integrity, growth and iron metabolism. However, not all effectors had an independent effect on virulence. This suggests a wide range of benefits to infection gained by the mobility of C. higginsianum effectors that likely interact in a complex way during infection.